The present invention relates to a quadrature type FM demodulator for outputting sound signals in use for a television receiver.
In FIG. 1 illustrating a prior phase detector device of this type, a filter 1 such as a ceramic filter is a band-pass filter of 4.5 MHz. A frequency modulated sound intermediate frequency (SIF) signal of 4.5 MHz is applied to the input of the filter 1. A series circuit of a bias resistor R1 and a bypass capacitor C1 is connected in parallel with the ceramic filter 1 at the output of the filter 1. A sound intermediate frequency integrated circuit (SIF-IC) designated by reference numeral 2 has two power source pins P.sub.C and P.sub.E and six signal pins P1, P2, P3, P4, P5 and P6. The SIF-IC 2 is comprised of an SIF limiter amplifier 3 including a plurality of differential amplifiers (not shown), and a phase discriminating type frequency modulation (FM) detector 4 including a double balance type differential amplifier. A series resistor circuit including two resistors R2 and R3 is connected between the signal pin P6 and ground. A capacitor C2 is connected between the signal pin P4 and a junction between the resistors R2 and R3. The coil L1, an inductance element, is connected between the signal pins P4 and P5. The capacitor C2 and the coil L1 cooperate to form a tuning circuit, i.e., resonance circuit 6 tuning to the sound carrier signal. The series resistor circuit 5 including the resistors R2 and R3 and the tuning circuit 6 including the capacitor C2 and the coil L1 cooperate to form a phase shifting circuit. A bypass capacitor C3 is connected between the signal pin P5 and ground. A capacitor C4 for deemphasis is connected between the signal pin P3 and ground.
In the FM demodulator device in FIG. 1, an SIF signal input is amplified by the limiter amplifier 3. An amplitude-modulated (AM) component is removed by the limited amplifier 3. The output signal of the limiter amplifier 3 is directly applied to a first input terminal of the FM detector 4. The output signal of the limiter amplifier 3 is also applied to the second input terminal of the detector 4 via a phase shifting circuit made up of the series circuit having the resistors R2 and R3 and the resonance circuit 6 having the capacitor C2 and the coil L1. The phase shifting circuit shift the phase of the output signal of the limiter amplifier 3 by 90.degree.. Therefore, the signal inputted to the second input terminal of the FM detector 4 (i.e. the phase shifted signal input terminal) is phase-shifted by 90.degree. from the signal inputted to the first input signal. Through this phase shift, the quadrature detection is executed by the FM detector 4 so that the sound signal is output from the third signal pin P3.
However, in the prior art phase detector devices, a number of signal pins (six pins in the above example) are required. This provides many technical restrictions in the process of its fabrication. For example, the sound signal detector needs one IC chip. This causes the manufacturing cost of the device to be quite high.